(1) Field of the Invention
The field of the invention is the gasification or conversion of organic matter into a gas mixture consisting primarily of carbon monoxide and hydrogen as well as other species. This disclosure relates to a highly flexible, multi-modal method, a system and an apparatus for generating a high quality syngas from the organic content of a wide variety of feedstocks in a gasification chamber incorporating a mechanical hearth, a method and apparatus for the beneficiation of a raw synthesis gas produced in a gasification reactor through homogenization, acceleration, preferential heating, reaction, and the conversion and removal of solids and liquid droplets, and systems incorporating such methods and apparatus.
(2) The Rationale for Gasification
There is growing interest in developing energy from sources other than those derived from petroleum and other fossil fuels. Gasification of the organic fraction of material such as biomass, coal, Municipal Solid Waste (MSW), Regulated Medical Waste (RMW), Construction and Demolition Debris (CDD), agricultural wastes, and various hazardous and non-hazardous industrial and commercial wastes can serve to provide clean, alternative energy. Although this invention is capable of processing any organic bearing material, an important focus is the utilization of waste as a feedstock. Currently, most waste is buried in a landfill, however, it is now well known that this method of disposal is undesirable, since landfills convey the threats of groundwater contamination and uncontrolled emission of greenhouse gases. Furthermore, the disposal of wastes in landfills squanders the opportunity for resource recovery—that is to recycle or reuse the materials, either through the separation of valuable components, or the conversion of the waste into energy or other useful products. In areas with very little available open space, such as densely populated islands, or regions with other geological constraints or impediments, landfills are not feasible options for waste disposal.
Existing Waste-to-Energy (WTE) systems attempt to recover some value from such waste streams through combustion. The waste is burned under excess oxygen conditions, producing primarily carbon dioxide (CO2), water vapor (H2O), and heat. The heat released by the incineration of waste is used to produce steam, or electric energy or a combination thereof. However the combustion based systems for WTE are inefficient, limited as to product type and, require substantially larger and more expensive air pollution control technology to comply with increasingly stringent environmental regulations due to substantially larger volumes of combustion gases when compared to gasification systems.
(3) Overview of Prior Art in the Field of Gasification
Systems operating under controlled sub-stoichiometric oxygen conditions, such as pyrolytic or controlled-oxidant gasification systems (collectively, ‘gasifiers’, ‘gasification systems’, or similar terms), offer significant advantages over landfill and combustion technologies for the conversion of wastes to useful products. In such systems, the organic fractions of a wide variety of feedstocks can be converted to a synthesis gas (‘syngas’) comprised primarily of hydrogen (H2) and carbon monoxide (CO). The syngas, when appropriately cleaned and conditioned, can be used to generate not only steam and electricity at higher efficiency than combustion based WTE systems, but also produce other commodities such as liquid fuels, hydrogen, or industrial chemicals. Gasification processes also offer the opportunity for the separation and sequestration of CO2; the sequestration of CO2 is not economically feasible with combustion-based WTE systems.
Modern gasification systems typically produce far lower pollutant emissions than combustion systems. Gasifiers are designed in variety of configurations, including moving bed, entrained flow, fluidized bed, and grate-based systems. Pyrolytic gasifiers typically do not involve the addition of gaseous reactants, and in non-pyrolytic, controlled-oxidant gasifiers reactants including oxidants such as air, oxygen, water, steam, or combinations thereof are introduced in a variety of ways. Gasifiers may be indirectly or directly heated. A number of gasifier designs have incorporated plasma arc torches as primary or auxiliary sources of heat.
It is generally known to those skilled in the art that gasifier performance and economics are optimized by maximizing the production of desirable species such as H2 and CO, while minimizing the production of fully-oxidized species such as H2O and CO2. It is also generally known that the performance and economics of downstream syngas utilization systems are optimized by minimizing the concentration of undesirable or unconverted species such as entrained solids, volatile organics, and tars in the syngas stream exiting the gasification system.
Typically, as gasifier temperature increases, organic particulate and gaseous species are more likely to react with injected reactant gases thus diminishing their concentration or virtually eliminating them. However there is a trade off in achieving optimum syngas quality since it is typical to increase gasifier temperature by oxidizing a portion of the valuable H2 and CO. As a result, there is a commensurate reduction of process efficiency. Thus, with the current state of the art the objective of achieving lower levels of undesirable particulate and complex organic species in the syngas involves sacrifice of process efficiency (or yield).
Some have attempted to optimize syngas properties and reduce the levels of undesirable particulate and complex organic species by increasing temperatures through the addition of supplemental heat sources, such as plasma torches, while controlling the availability of oxidants. Two recent examples of such approaches are disclosed in U.S. Patent Application Nos. 2007/0266633 (Tsangaris et al.) and 2009/0077887 (Michon et al.).
Tsangaris discloses a “gas reformulation chamber” incorporating one or more plasma torches and one or more oxygen sources. In this system “the plasma torches heat the chamber and the input gas is thereby converted to reformulated gas” and “the gas reformulating system uses torch heat from a plasma torch to dissociate the gaseous molecules thereby allowing their recombination into smaller molecules . . . ”. Within this gas reformation chamber, “plasma torch power is adjusted to stabilize the reformulated gas exit temperatures at the design set point. In one embodiment, to ensure that the tars and soot formed in the gasifier are fully decomposed the design set point is about 1000° C.”
It is apparent from this description that the Tsangaris design is intended to raise the temperature of the entire syngas stream to achieve the temperatures desired for the conversion of tars and soot (solid particulates).
Michon discloses an elongate reactor containing a plasma torch which generates a plasma plume. Within this reactor, the raw syngas is fed “so that the flow of syngas encounters said plasma jet at least partially so as to mix said syngas and plasma jet”. The plasma jet is “seeded with . . . radicals having high chemical reactivity.” The radicals react with “the non-advantageous molecules of the syngas to be treated brought up to temperature.” The Michon invention further creates a “thermal or thermochemical transformation zone” resulting “from the intimate mixing of the syngas to be treated and of the plasma jet”. Thus, the design is intended to directly mix the syngas stream into the plasma plume, in order to bring about gasification reactions between undesirable (and typically dilute) species in the syngas stream with reactive species present in the plasma plume. This intimate mixing is intended to achieve “syngas/plasma jet mixture temperatures that are higher than with prior art apparatus.” The apparatus of Michon is equipped with a sensor “for measuring the temperature of the outlet gas in a manner such as to adjust the quality of the outlet gas”.
Michon further discloses a “cold wall, low velocity wall” reactor design, wherein the walls of the elongate reactor are protected from temperature and erosive wear by, among other means, “feeding in a protective fluid tangentially to the wall . . . said fluid being at ambient temperature.”
It should be noted that the plasma jet envisioned by Michon is volumetrically small relative to the syngas stream “with the plasma jet having a diameter d, the inlet port has a feed orifice for feeding in said syngas whose diameter D is such that D/d is greater than or equal to 10”. Those skilled in the art know that, although industrial plasma torches capable of producing large amounts of heat (exceeding 1 MW of power) are available, the plume of a plasma arc torch is of necessity limited in size by the constraints of plasma arc torch design. As a result, the desired intimate mixing between a small plasma arc plume and large volumes of syngas may be difficult to achieve at large commercial scale. Thus, the achievable sizes and capacities of the Michon reactors may be limited by the plasma plume size, and commercial scale gasification systems may require a multitude of Michon reactors. This apparent limitation may result in high capital and operating costs, rendering such systems commercially unviable.
Looking specifically at the aforementioned undesirable species in the syngas, they fall into three general categories: complex organic species in the gaseous phase (“volatile organics”), complex organic species in the liquid particulate phase (e.g., “tars”), and solid particulates (which may contain carbonaceous and/or inorganic materials). As noted above, the rate of conversion of these materials to desirable H2 and CO is affected by the temperature of the reacting species. The rates of reaction are further affected by the availability of oxidants, adequate mixing of the reacting species, reactive particulate surface area (in the case of the particulate species), and sufficient reaction time. Generally, the result of such influences is that the gaseous species react more rapidly than the liquid phase particulates, and the liquid phase particulates react more rapidly than the solid phase particulates, when all such species are at the same temperature.
Prior art systems including those of Tsangaris and Michon envision “bulk heating” systems wherein the desirable syngas components, volatile organics, and particulate species are simultaneously heated to the temperatures required for the conversion of the most problematic of the undesirable species—the solid carbonaceous particulates. While elevating the temperature of the entire—syngas stream can promote the conversion of volatile organics and particulates, this approach can also result in the consumption of desirable H2 and CO. Furthermore, this approach can require significant energy input, as large quantities of already-converted gas must be heated to raise the temperature of the typically small concentration of unconverted particulates. An approach which can preferentially elevate the temperature of the more problematic particulate species and thus promote their conversion without significantly heating the bulk of the syngas stream would advantageously require less energy input.
It is therefore desirable, in view of the limitations of existing gasifier technology, to develop a gasification system which can maximize the levels of desirable CO and H2 in syngas and minimize the levels of CO2, H2O, tars, volatile organics, and solids, without requiring excessive energy consumption or excessive numbers of plasma arc torches. None of the prior art discloses such a system. The present invention achieves this optimal design condition, thus representing a significant advancement beyond, and improvement over prior art.